Hard to believe, but summer ends soon: on 22 September, the day of the autumnal equinox. Sunset keeps getting noticeably earlier, and sunrise later—and we won't turn our clocks back from "summer" time for a few weeks yet—but when the Sun crosses the equator headed southward on the equinox, summer is unambiguously over. It is fitting that summer ends with its most iconic star group, the Summer Triangle, high overhead in the early evening. The chart above (see image) is for 8:30 p.m. on the evening of the equinox, but it will be fine for early evening viewing for several weeks on either side of that date. The three bright stars of the Summer Triangle, Vega, Altair, and Deneb, are easy to pick out, even in city lighting or bright moonlight. Each of them is in a different constellation: Lyra, Aquila, and Cygnus respectively. (Those constellations are not shown in the figure above to keep the graphics simpler.) The Triangle itself is not technically a constellation, but an unofficial, yet handy, grouping called an asterism. The Big and Little Dippers are actually asterisms too, being subsets of the official constellations of Ursa Major and Ursa Minor respectively.
In the west it is easy to pick out Arcturus, the second brightest star in the night sky, which anchors the constellation Boötes, the Herdsman. East of the Summer Triangle, look for the "square" of stars that are the most notable feature of Pegasus. The star Alpheratz occupies the corner of the square that connects to the constellation Andromeda. And scanning northward from Andromeda takes us by the "W" shape of the constellation Cassiopeia.
Now that you can find your way around the late summer and early autumn evening sky, try it again from a dark location (preferably with a clear horizon) on a moonless night. You'll have a great view of our own Milky Way Galaxy, which arches from northeast to southwest, passing behind Cassiopeia and the Summer Triangle. (see image) When your eyes have become accommodated to the darkness, you will be able to see that the Milky Way becomes noticeably wider and richer as it approaches the star fields of Sagittarius and Scorpius in the southwest. That widening is the "bulge" of stars that makes up the inner parts of the galaxy, and it is crossed by the dark dust lanes of the "disk" of our galaxy, which we see silhouetted against the brighter bulge stars. In a dark sky it's an amazing sight when you realize you can see half of our entire home galaxy spread, as if in cross-section, across the whole sky, on these late summer evenings.
From the University of Wisconsin's "Space Place"
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